1. Field
The embodiments herein relate to the technical field of medical imaging, particularly MR cardio imaging, although it can find application in any field where there is the need to quantify flow in a moving object such as in non destructive testing applications.
2. State of the Art
The accurate study and characterization of blood flow patterns and pathophysiology in the cardiac valves and in the main vessels of the human anatomy play a role of primary importance in the diagnosis and treatment of cardiovascular dysfunctions.
Stenosis, inlet and outlet valve regurgitation or congenital defects represent few examples in which the cardiovascular function needs a close imaging follow-up to assess the severity of the symptoms and the consequent optimal timing and type of surgical intervention.
One of the most used techniques to analyze blood flow in clinical setting is flow sensitive Magnetic Resonance (MR) imaging.
The intrinsic sensitivity of MR to flow allows reliable quantification of vascular hemodynamics and qualitative delineation of flow patterns, without the restriction to anatomic coverage or flow directions. This is normally performed by the acquisition of a number of 2D phase-contrast Magnetic Resonance Imaging (MRI) planes, also known as 2D MR Flow.
However, this method requires careful planning of the 2D phase-contrast MR image acquisition planes by an MR operator. For example, for each valve that is being assessed, the operator needs to carefully position a plane that is used for acquisition. The planning of this plane is of great importance because this plane is static and needs to be placed perpendicular to the blood flow. The fact that the acquisition plane is static means the plane does not move during the entire acquisition of the data. Even if the operator manages to place the plane optimally within the dataset, certain problems still arise. For instance, out of plane motion will occur. This out of plane motion (through plane motion i.e., motion in the longitudinal direction through the acquisition plane positioned at the heart valve of interest) is a result of movement of the heart during the cardiac cycle. This is a major obstacle for accurate flow estimations, especially in transvalvular regions as reported by previous studies such as Kayser et al., “MR velocity mapping of tricuspid flow: correction for through-plane motion,”J Magn Reson Imaging 1997; 7: 669-673. This through plane motion results in unusable data hence data is acquired in the static plane but this plane does not contain the correct features (i.e. the valve) anymore. Therefore, this also results in the inability to make a diagnosis based on this data.
Furthermore, due to this high operator dependency, incorrect planning occurs regularly—even in high patient volume centers—, making clinical assessment difficult, and potentially hampering a correct diagnosis.
Furthermore, the operator needs previous knowledge of the flow-encoding direction in order to position the acquisition plane perpendicular to the blood flow. All these aspects result in a time-consuming and error prone process.
Time resolved three dimensional phase contrast MRI (4D MR flow) is an evolving imaging technique used for evaluation of multidirectional flow velocity data. In 4D MR flow, anatomical and three-directional velocity information are acquired for each voxel within a 3D isotropic volume over time.
This type of data allows the analysis of the blood flow from any spatial oriented plane. Therefore, the aforementioned problems of a static analysis plane does not hold anymore, since the plane can be repositioned at each acquired time point (i.e. at each acquired phase during the cardiac cycle) to tightly follow the cardiac and respiratory movements. Due to the cardiac and respiratory movement compensation and by centering the analysis plane for the object of interest, through plane motion will therefore not occur anymore as taught by Westenberg et al., “Accurate and Reproducible mitral valvular blood flow measurement with three-directional velocity-encoded magnetic resonance imaging,”J Cardiovasc Magn Reson., 2004; 6:767-776.
Repositioning of the analysis plane during the cardiac cycle requires tracking of anatomical landmarks. A downside of the 4D MR Flow imaging technique is the limited signal to noise ratio of the anatomical data. This results in poor detailed outlining of anatomical structures. Therefore, a different strategy has to be adopted to efficiently track anatomical structures movements.
Several authors addressed the problem by manually identifying the valve position on each time frame on two additional MR sequences, for instance Westenberg et al., “Mitral valve and tricuspid valve blood flow: accurate quantification with 3D velocity-encoded MR imaging with retrospective valve tracking,” Radiology 2008; 249:792-800 and Roes et al., “Flow assessment through four heart valves simultaneously using 3-dimensional 3-directional velocity-encoded magnetic resonance imaging with retrospective valve tracking in healthy volunteers and patients with valvular regurgitation,” Invest Radiol 2009; 44: 669-674. Their method requires two additional long axis cine MR acquisitions acquired orthogonal to each other and intersecting with the valve of interest, e.g. for the mitral valve (as shown in FIG. 2) a left ventricular two-chamber and four-chamber cine MRI acquisition are required. That is for each valve, two long axis cine datasets have to be acquired. Accurate planning of these long axis cine images is needed to avoid out of plane motion of the valve of interest.
Furthermore, this approach of manual annotation of valve locations during the cardiac cycle has two important limitations. Manual annotation makes the tracking results user dependent and scarcely reproducible, and also results in large intra- and inter observer variations.
The second limitation of manual annotation of valve location is the long process time required for each case. Considering these limitations, manual valve tracking is impracticable for clinical routine.
In another work Dewan et al., “Deformable Motion Tracking of Cardiac Structures (DEMOTRACS) for Improved MR Imaging”, IEEE Conference on Computer Vision and Pattern Recognition, 2007, the cardiac motion information was estimated by valve tracking in MR images acquired during an initial scan (Long Axis views), which can then be used to adaptively re-position the acquisition slice during 2D MR Flow acquisition.
The tracking method is based on a predefined database of manual identified anatomical landmarks. This method requires a database with identified anatomical landmarks for specific scan orientations (long axis views). This makes this method heavily dependent on the content of the predefined database, acquisition method and long axis plane orientation.
Further, this work focuses on valve tracking for dynamic acquisition plane positioning for 2D MR Flow acquisitions only. This method is applied during a cardiac MR flow acquisition, with the patient inside the scanner. Errors in the valve tracking result in incorrect flow acquisitions and no diagnosis will be possible afterwards.